1. Field of the Invention
This invention relates to electronic pulse generation circuit and more particularly to pulse generation circuits that have a random interval between pulses.
2. Description of Related Art
It is not possible to produce truly random sequences of numbers in deterministic system such as logic circuits or computer systems. However, the generation of pseudo random sequences of numbers, either decimal or binary is well known in the art. A common method for the generation of random numbers is the additive congruential method. The additive congruential method employs the following formula to generate the random number: EQU a[k]=(a[k-b]+a[k-c])mod m
where:
a[k] is the k.sup.th digit of a sequence of digits PA1 a[k-b] is the b.sup.th previous digits of the sequence of digits. PA1 a[k-c] is the c.sup.th previous digits of the sequence of digits. PA1 MOD m is the modulus of the number system for the sequence of numbers. In the case of a binary system, MOD m is equal to 2.
This formula can be implemented as a linear feedback shift register as shown in FIG. 2. The linear feedback shift register consists of the flip-flops U1, U2, U3, U4 and U5. The output of the flip-flops U1, U2 U3, and U4 are connected respectively to the inputs of the flip-flops U2, U3, U4, and U5. The output of the flip-flop U5 is connected to the first input of the exclusive-OR U6. The second input of the exclusive-OR U6 is connected to the output of the flip-flop U2. The output of the exclusive-OR U6 is connected to the input of the flip-flop U1. The clock CLK provides the signal necessary to sequentially transfer the signal at the input of the flip-flop U1 through flip-flops U2, U3, and U4 to the flip-flop U5. The outputs Q0, Q1, Q2, Q3, and Q4 of the flip-flops U1, U2, U3, U4 and U5 form a pseudo random binary number. The low order bit is formed by the output Q0 of the flip-flop U1 and the high order bit is formed by the output Q4 of the flip-flop U5. The circuit as shown produces the following sequence at each cycle of the clock CLK:
______________________________________ Q4 Q3 Q2 Q1 Q0 ______________________________________ T0 1 1 1 1 1 T1 1 1 1 1 0 T2 1 1 1 0 0 T3 1 1 0 0 1 T4 1 0 0 1 1 T5 0 0 1 1 0 T6 0 1 1 0 1 T7 1 1 0 1 0 T8 1 0 1 0 0 T9 0 1 0 0 1 T10 1 0 0 1 0 T11 0 0 1 0 0 T12 0 1 0 0 0 T13 1 0 0 0 0 T14 0 0 0 0 1 T15 0 0 0 1 0 T16 0 0 1 0 1 T17 0 1 0 1 0 T18 1 0 1 0 1 T19 0 1 0 1 1 T20 1 0 1 1 1 T21 0 1 1 1 0 T22 1 1 1 0 1 T23 1 1 0 1 1 T24 1 0 1 1 0 T25 0 1 1 0 0 T26 1 1 0 0 0 T27 1 0 0 0 1 T28 0 0 0 1 1 T29 0 0 1 1 1 T30 0 1 1 1 1 ______________________________________
The sequence repeats continuously. It should be noted that the sequence can have no zero value or it ceases to operate. Additionally, the circuit as configured produces the binary numbers available only once within a cycle of the sequence of numbers and is therefore, not really pseudo random. This method though is generally adequate for many practical applications.
U.S. Pat. No. 5,680,516 (Kadowaki et al.) describes a multiple pulse series generating device that incorporates a linear feedback shift register and a data providing logic circuit to adjust the probability density of the multiple pulse series.
U.S. Pat. No. 5,563,573 (Ng et al.) discloses a pseudo random switched resistor that emulates a relatively high impedance. The control of the switched resistor is a pseudo random pulse generator. The pseudo random pulse is a linear feedback shift register similar to that described in FIG. 2.
U.S. Pat. No. 4,578,598 (Faulhuber) teaches a random pulse generator circuit that incorporates a linear feedback shift register similar to that described in FIG. 2. A digital-to-analog converter, a voltage-to-frequency converter, and circuitry provides the random sequence of output pulses. The pulse duration, the pulse interval, the pulse repetition frequency, and the degree of randomness of the pulse repetition frequency are selectable.
U.S. patent application Ser. No. 09/048,381 (Yap et al.) filed, Mar. 26, 1998 for "A Pseudo-Random Telemetric Data Communication System" and assigned to the same assignee as this invention, requires that telemetric data be transmitted in pseudo random intervals from a transmitter to a telemetric data receiver. The pseudo random intervals are sufficiently separated that multiple transmitters may send telemetric data, while minimizing the probability of collisions between data transmissions. FIG. 1 shows a representative plot of an output of the pseudo random sequence generator that determines the timing sequence of the transmission of the telemetric data.
The pseudo random sequence generator has an inactive period of time Tia where the pseudo random sequence generator produces no pulses. At the end of the inactive period of time Tia, the active period of time begins. A number of pulses P.sub.1, P.sub.2, . . . , P.sub.n is generated. The interval between each pulse Tp is pseudo randomly determined.
The randomness of the transmissions of the telemetric data has to be sufficient that the telemetric data is transferred with minimal collisions from other telemetric sources.